SINGLE-CRYSTAL EPR STUDY OF Fe3+ AND VO2+ IN PREHNITE FROM THE JEFFREY MINE, ASBESTOS, QUEBEC

Abstract

Prehnite from the Jeffrey mine, in Asbestos, Quebec, has been investigated by optical microscopy, electron-microprobe analysis, single-crystal X-ray-diffraction (XRD) analysis, and electron paramagnetic resonance (EPR) spectroscopy. Electron-microprobe analyses yield an average iron content of 0.053(34) wt% Fe 2 O 3 ; single-crystal XRD intensity datasets confirm the space group P 2 cm . Single-crystal EPR spectra measured from 290 to 13 K reveal at least six paramagnetic defects: three Fe 3+ centers, a VO 2+ radical, a Mn 2+ center, and an Al-associated O − center. Spin-Hamiltonian parameters of the VO 2+ radical show that it resides at an octahedral site and most likely originated from a V 3+ O 4 (OH) 2 octahedron by removal of a proton and trapping of a hole to form a covalent V=O bond during natural radiation. Spin-Hamiltonian parameters, including the high-spin terms of type S 4 , BS 3 and BS 5 , for the dominant Fe 3+ center also show it to reside at an octahedral site. Two less abundant Fe 3+ centers have similar orientations in effective principal axes of g, indicative of occupancies at the octahedral site as well. Their differences in effective principal values of g may be attributed to different degrees of local distortion of the octahedral site. A previous interpretation of the powder EPR spectra for two Fe 3+ sites in prehnite has been shown to be incorrect. Spin-Hamiltonian parameters for Fe 3+ in prehnite have wide implications for interpretation of EPR spectra of Fe 3+ in layer silicates.